1. Field of the Invention
The present invention relates to a semiconductor apparatus and a method for inspection thereof, and particularly to a light-receiving element having a photodiode unit and a method for inspecting a common mode rejection thereof.
2. Description of Related Art
In a photocoupler, a light-emitting element and a light-receiving element are disposed to oppose each other. Usually, a photocoupler has a function to electrically insulates two circuits with different potentials and transmit a signal via light. On the other hand, a photocoupler can be considered as a capacitor which uses the light-emitting and light-receiving sides as electrodes. Therefore, the photocoupler has a stray capacitance Cf between the light-emitting side and the light-receiving side. If a rapidly changing voltage (noise) dv/dt is applied between an input and an output of the photocoupler, a displacement current (I=Cf·dv/dt) flows, thereby generating noise (hereinafter referred to as “electric noise” as appropriate) in the output of the photocoupler. This noise causes malfunction. As one of the indices to indicate the capability to maintain a correct output state, the Common Mode Rejection (CMR) is used. The higher the CMR, the noise resistance is evaluated to be higher. If a steep voltage is applied between the input and output depending on the user environment, a photocoupler of high CMR needs to be selected.
FIG. 5 illustrates a pattern example of a light-receiving element according to a related art. The light-receiving element 80 of FIG. 5 includes a photodiode unit (hereinafter referred to as “PD unit”) 81, pads 82-1 to 82-3, and test pads 83-1 to 83-3. In response to a light entered to the PD unit 81, the light-receiving element 80 converts the light into an electric signal. However, electric noise is also input to the PD unit 81 along with the light. Therefore, the PD unit 81 needs to separate the light from the noise. In order to remove the electric noise, a low-resistance shield film (not shown in FIG. 5) is formed in the upper part of the PD unit 81. For example, Japanese Unexamined Patent Application Publication No. 4-354379 discloses a technique concerning a photocoupler having a light-receiving element. The light-receiving element is provided with measures for the shield film by a polysilicon layer in the light-receiving unit of the light-receiving IC. A polysilicon film is used as the shield film.
The mechanism to remove noise by the shield film is explained hereinafter. FIG. 6 shows a pattern diagram of an example of the PD unit cross-section surface of the light-receiving element of a related art. A PD unit 90 includes a shield film (low-resistance shield film) 91, an interlayer film 92, a base region 93, and an epitaxial region 94. The base region 93 and the epitaxial region 94 play a role of the photodiode 95. The shield film 91 is connected to the ground (GND). The electric noise (indicated by the dotted line in FIG. 6) input to the PD unit 90 is removed by this shield film. Accordingly, the shield film 91 allows the light entered to the PD unit 90 (indicated by the solid line in FIG. 6) to transmit therethrough and divert the electric noise to the ground, so that malfunction in the output can be prevented. Therefore, the light-receiving element must be able to remove the noise.
As mentioned above, CMR is used as one of the indices to show the capability to maintain the correct output state, which is the capability to remove noise that causes the malfunction. As shown in FIG. 6, when the shield film removes noise, CMR depends on the sheet resistance value of the shield film. In order to guarantee CMR of a photocoupler, it is desirable to measure CMR for each photocoupler in the photocoupler manufacturing process. The measuring method of CMR is explained hereinafter.
FIG. 7 shows an example of a CMR measuring circuit.
FIG. 8 shows an example of a measurement result. In the CMR measuring circuit, a transistor 72 formed in the light-receiving element of the output is connected to a load resistance (RL). When a pulse (VCM) with a steep slope (VCM/μs) is applied between an input and an output terminal (between P1 and P2) while the power supply voltage (Vcc) is applied, CMR is observed as a change in a voltage (Vo) between a collector and an emitter of the output transistor 72. CMR indicates a maximum voltage change (VCM/μs) in which the photocoupler does not malfunction by the noise. A pulse of 10 kV or more is applied to evaluate CMR. More specifically, to measure CMR, a malfunction (Vr2 in FIG. 8) in a falling pulse is measured while an input forward current IF to a light-emitting diode 71 exists, and a malfunction (Vr1 in FIG. 8) in a rising pulse while the input forward current IF to the light-emitting diode 71 does not exist. CMR is defined as the maximum voltage change (VCM/μs) of the largest VCM in which Vo change is predetermined level or below.
CMR of a photocoupler is generally required to be 10 kV/μs or more. Thus a pulse of 10 kV/μs or more must be applied to measure the CMR. CMR cannot be measured by a normal electrical properties evaluation equipment in an assembly line, and a special simple measuring device as shown in FIG. 9 is used.
The measuring method is that a photocoupler is mounted to a test board 61, a pulse is applied by a high-pressure equipment 62 while checking an oscilloscope 63 so as to check a change in the output waveform. However, for each of the photocoupler, this method requires tasks of mounting the photocoupler to the test board 61, applying a pulse, checking waveform, and demounting. Accordingly, this method takes time. Therefore, it is difficult to evaluate all photocouplers and necessary to manually check by sample. A dedicated automatic evaluation equipment can be implemented but it will be a large business investment.
As described so far, a photocoupler has a stray capacitance between the light-emitting and light-receiving side. Thus malfunctions caused by noise must be prevented. The quality of the photocoupler can be indicated by showing the capability to remove noise using the CMR properties. However, as a predetermined voltage is required to measure CMR of a photocoupler, it is difficult and also costly. Therefore, only a limited number of products can be measured.